This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The fast uptake of the Third Generation Partnership Project (3GPP)-Long Term Evolution (LTE) in different regions of the world shows both that demand for wireless broadband data is increasing, and that LTE is a successful platform to meet that demand. The existing and new spectrum licensed for exclusive use by International Mobile Telecommunications (IMT) technologies will remain fundamental for providing seamless coverage, achieving the highest spectral efficiency, and ensuring the highest reliability of cellular networks through careful planning and deployment of high-quality network equipment and devices.
In order to meet the ever increasing data traffic demand from users, particularly in concentrated high traffic buildings or hot spots, more mobile broadband bandwidth will be needed. Given a large amount of spectrum available in unlicensed bands around the globe, the unlicensed spectrum is more and more considered by cellular operators as complementary means to augment their service offering. While the unlicensed spectrum can never match the qualities of the licensed regime, solutions that allow an efficient use of it as a complement to licensed deployments have a potential to bring a great value to 3GPP operators, and ultimately to the 3GPP industry as a whole. This type of solutions would enable operators and vendors to leverage the existing or planned investments in LTE/Evolved Package Core (EPC) hardware in radio and core networks.
Recently, an emerging technology, referred to as Licensed-Assisted Access (LAA), has become a hot topic in 3GPP studies. An LAA framework builds on carrier aggregation solutions introduced in LTE Release-10 to access the additional bandwidth in the unlicensed spectrum. In an LAA system, an access point (AP) may serve user equipment (UE) via both a licensed carrier and an unlicensed carrier. The licensed carrier maintains a radio connection, while the unlicensed carrier is mainly used for data rate boost. In the LAA system, transmissions on the licensed carrier may be performed based on resource scheduling at the AP according to specifications as defined for LTE systems, while transmissions on the unlicensed carrier may follow the listen before talk (LBT) mechanism as defined for WiFi systems.
The LBT mechanism is widely applied in WiFi systems aiming to avoid collisions between neighboring radio links. FIG. 1 shows an example of the LBT mechanism as defined in IEEE (Institute of Electrical and Electronics Engineers) 802.11. According to the LBT mechanism, when there is data for transmission from a transmitter to a UE, the transmitter shall first sense a channel for a radio link directed to the UE for a certain time period, e.g. for DIFS (Distributed Coordination Function (DCF) inter-frame space), PIFS (Point Coordination Function (PCF) inter-frame space) or SIFS (Short Inter-frame Space). If the channel is sensed to be busy, then the transmitter shall wait e.g. for a Defer Access period as shown in the figure. If the channel is sensed to be idle, then the transmitter may generate a random back-off time period, which may be referred to as a contention window or a back-off window, as shown in the figure. The channel may be determined to be available for data transmission only if the channel is sensed to be idle during the whole back-off window. Once the channel is determined to be available, the transmitter may start to transmit data over the channel to the UE.
In order to improve system performance of WiFi systems, the beamforming technology has been introduced into the WiFi systems. With high gain beamforming, directional channel sensing with the conventional LBT has been investigated so as to improve spatial multiplexing. As such, a transmitter may perform channel sensing for different directions so as to determine channel availability for respective links in these directions.
In terms of complexity control, analog beamforming antennas are preferably adopted to implement beamforming. For analog beamforming, a precoding matrix is applied by using analog phase shifters after digital-to-analog (DA) conversion. Thus, usage of analog beamforming antennas may reduce the number of Analog-to-Digital (AD)/DA converters. Since the cost of AD/DA converters is very high and power efficiency may also form a challenge when too many AD/DA converters are applied, analog beamforming antennas are preferred in implementation of beamforming. However, an analog beamforming antenna can provide a very limited number of Tx/Rx Radio Frequency (RF) chains, while one Tx/Rx RF chain can only generate one main beam and it is impractical to use the same RF chain to generate multiple main beams for multiple users simultaneously since the users are usually located in different directions. For an AP equipped with an analog beamforming antenna, the number of beams that can be generated by the AP is up to the number of TX RF chains of the AP antenna and the number of directions that the AP can sense is up to the number of RX RF chains of the AP antenna.
For LAA systems, when one AP equipped with an analog beamforming antenna serves multiple users over a licensed carrier and a shared unlicensed carrier, the AP may need to perform directional channel sensing to determine channel availability before data transmission to each user on the unlicensed carrier. With the limitation of Rx RF chains, the directional channel sensing may have to be performed sequentially and meanwhile complies with the LTE frame structure used on the licensed carrier, which may cause a large overhead and resource waste, as explained with reference to FIG. 2, FIG. 3A and FIG. 3B.
FIG. 2 shows an LTE frame structure as used on the licensed carrier. As shown, an LTE radio frame has 10 LTE subframes, while each subframe may have 14 symbols. Usually, the first three symbols may be used for channel sensing, after which data transmission may be performed.
FIG. 3A shows an example of one AP serving three UEs by establishing three radio links L1, L2 and L3 in different directions. It is assumed that the AP equipped with an analog beamforming antenna has a single RX RF chain to serve the three UEs and can sense channels for links L1, L2 and L3 sequentially.
According to the existing directional sensing solution, the AP may have to perform channel sensing for the served UEs one by one. If the AP performs channel sensing for the three UEs (i.e., UE1, UE2 and UE3) sequentially due to the limitation of the RX RF chain at the AP side, the AP may only complete the channel sensing for one UE within one subframe. In this case, if a channel sensed in one subframe is determined to be busy for a UE, then that subframe may not be used for channel sensing of other UEs, although the channel may be available for the other UEs in other directions. FIG. 3B shows an example of a channel sensing sequence, in which mark “X” indicates that the channel being sensed is busy and mark “✓” indicates that the channel being sensed is idle. As shown, even though the channel for UE 3 may be idle, it still has to wait for two subframes for sequentially sensing the channels of UE 1 and UE 2 before data can be transmitted to UE 3. Accordingly, the efficiency of resource utilization may be low.
Therefore, there is a need for a more efficient sensing solution applicable for a network involving resource sharing, especially for LAA networks.